1. Field of the Invention
The present invention relates to a sample holder and a charged particle beam apparatus using the same, suitable for applying a magnetic field to a sample to be observed or worked.
2. Description of the Related Art
When an electron beam passes through a magnetic material sample placed in an electron microscope, electrons are deflected by Lorentz force caused by magnetic fluxes inside the sample. By Lorentz electron microscopy and electron beam holography which apply this principle, it is possible to observe magnetic domain structures inside a sample. This method is most effective in investigating the magnetic response of an in-plane component of magnetization of a sample upon applying a magnetic field to the sample along an in-plane direction.
By incorporating a magnetic field application element provided with a magnetic circuit including a magnetic core made of a soft magnetic material and a coil for magnetic field application into a sample holding device, an “in-situ” observation of a change in-plane magnetic domain structure of a sample is performed during application of a magnetic field perpendicular to an optical axis. Samples for electron microscopes generally have an external form like the shape of a disc with a diameter of 3 mm. While it is possible to accurately position a sample inside a magnetic gap that is larger than the sample diameter and apply a magnetic field along a direction parallel to the plane of the sample, a maximum magnetic field is not more than several tens of oersteds in most cases. Accordingly, in order to increase a magnetic field applied, it is practiced to bring the surface of a sample in contact with the top and under side surfaces of a pair of magnetic poles having a magnetic gap whose width is narrower than the size of a sample (Japanese Published Unexamined Patent Application Nos. Hei 8-264146 and 2007-80724). In this case, however, because of making use of a magnetic field that leaks from the magnetic gap into free space, the magnetic field applied to the sample does not become completely parallel to the surface of the sample and has a component in a direction perpendicular to the surface of the sample. Because the point where the magnetic field is produced is quite far from the center of the magnetic gap, there is a disadvantage that the applied magnetic field is subject to a large spatial variation. Another disadvantage of this method is that the magnetic field does not become zero even if conduction current is zeroed because of remnant magnetization of the magnetic material and it is difficult to observe the state of the sample in a condition where there is no magnetic field.
Even in a case where this method is used, a maximum applied magnetic field is not more than about 200 oersteds. This is because this mechanism makes it difficult to accurately position a sample to be aligned with the magnetic gap and, accordingly, the magnetic gap remains on the order of 1 mm.
There is also an omni-directional type magnetic field application device including a set of superconductive electromagnets installed within a main body so as to surround a sample holding device (Japanese Published Unexamined Patent Application No. 2002-296333). Because of using superconductive coils of an air core type, this magnetic field application device is capable of putting remnant magnetization to zero, but a maximum magnetic field is not more than about 200 oersteds. The magnetic field application device having a five-tier structure and provided with a mechanism for correcting the deflection of an electron beam caused by a magnetic field has a disadvantage that its shape becomes too large.